In accordance with a trend toward higher Large Scale Integration (“LSI”) densities, the input voltages of LSIs are being lowered more and more. Therefore, the number of applications for generating low output voltages has increased for power supply circuits for outputting voltages to be input to LSIs. A conventional switching-type, step-down type power supply circuit is shown in FIG. 1. When a switching device SW1 is on, the inclination of a change in the current of an inductor L1 is represented by
                              [                      Expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            ⅆ                          I              L                                            ⅆ            t                          =                                                            V                IN                            -                              V                OUT                                      L                    .                                    (        1        )            In a case wherein an input voltage VIN to a power supply circuit is high while an output voltage VOUT is low, an inductor current IL rises sharply. For example, there may be a case wherein the input voltage VIN is 5 V, 12 V, 48 V, etc., and the output voltage VOUT is 1 V or lower. On the other hand, when the switching device SW1 is off, and a switching device SW2 is on, the inclination of the change in the current of the inductor L1 is represented by
                              [                      Expression            ⁢                                                  ⁢            2                    ]                ⁢                                                                                                                          ⅆ                              I                L                                                    ⅆ              t                                =                      -                                          V                OUT                            L                                      ,                            (        2        )            and since the output voltage VOUT is low, the inductor current IL is reduced with a gentle inclination.
Assume a case of the occurrence of sudden reduction of a load current ILOAD that flows through a load, to which the output voltage VOUT of a step-down type power supply circuit using the above described inductor is applied. For example, when the inductor current IL is not reduced at the time that the load current ILOAD is dropped from 1 A to 100 mA, an extra charge is supplied to an output capacitor COUT, and causes overshoot, which is a rise in the output voltage VOUT that exceeds a predetermined threshold value. Even when the switching device SW1 is turned off, the inductor current IL is only gently reduced, as explained above in association with expression (2). Therefore, a capacitor having a large capacitance must be employed, so that the rise in the output voltage VOUT due to the inflow of the inductor current IL occurs only within a predetermined range. Here, the most sudden reduction in the load current ILOAD is a case wherein the load current ILOAD is reduced almost at the same time as when a circuit, such as an LSI, connected as a load, is turned off, and the least sudden reduction is a case wherein the load current ILOAD is reduced at least faster than the inductor current IL in expression (2). In this case, an extra charge is supplied to the output capacitor COUT, and overshoot may occur.
Further, when a series regulator, such as an LDO, is employed as a power supply circuit, the output voltage VOUT is increased in a case wherein the load current ILOAD is suddenly reduced, and a period wherein the output voltage VOUT is being increased is determined based only on the frequency range of the power supply circuit. However, when a power supply circuit is a switching regulator employing an inductor, upon the occurrence of a sudden reduction in the load current ILOAD, a surplus current occurs in the inductor and an extra charge is supplied to the output capacitor COUT, so that a period in which the output voltage VOUT is being increased is extended until an extra charge has been consumed by the load, which especially becomes a problem.
A power supply circuit that can quickly respond to a reduction in a load current is disclosed in patent literature 1, and is shown in FIG. 2. A power supply circuit 1 includes a control circuit 10a and a converter 20a, which includes an output transistor Q1, which is an N-channel MOS transistor, a diode D1, a choke coil L1 and a smoothing capacitor C1. When the output transistor Q1 is turned on or off based on a signal SG1 output by the control circuit 10a, an input voltage VIN is stepped down, and a stepped-down voltage is output as an output voltage Vo to a load connected to an output terminal To. The output voltage Vo is adjusted to reflect a target value that is predetermined by changing a ratio of the ON time to the OFF time of the output transistor Q1.
The output terminal To is connected to an input terminal T1 of the control circuit 10a. The input terminal T1 is grounded via resistors R1 and R2. A junction point between the resistors R1 and R2 is connected to the inverting input terminal of a comparator 11, and with this arrangement, the output voltage Vo is divided by the resistors R1 and R2, and a divided voltage V1 is input as a feedback signal to the inverting input terminal of the comparator 11. The feedback signal is compared with a voltage reference Vr by the comparator 11, and based on the comparison results, the output transistor Q1 is turned on or off. According to this technique, in order to suppress low-frequency oscillation, which tends to occur in a case wherein the ON time or OFF time of a switching device is extremely extended, a voltage obtained by adding a slope signal to a reference voltage Vref is employed as the voltage reference Vr, which is used for comparison with the feedback signal. As a result, a period required after the load is suddenly changed until the output voltage Vo converges on the target value, based on the reference voltage Vref, can be reduced, and accordingly, the occurrence of overshoot can be suppressed. At this time, the reference voltage Vref is set, so that when the output voltage Vo reaches a standard value, the reference voltage Vref matches the divided voltage V1 obtained through the resistors R1 and R2.
A generally well known method for controlling a switching-type power supply circuit is a method for comparing the output voltage with a predetermined voltage, and for turning on a switching device in a case wherein the output voltage is equal to or lower than the predetermined voltage, and in addition, there are various specific control forms, including the prior art described in patent literature 1.